Introduction to ENVIRONMENTAL TOXICOLOGY Impacts of Chemicals Upon Ecological Systems - CHAPTER 11

CHAPTER 11 Ecological Risk Assessment INTRODUCTION A great deal of environmental toxicology is performed with the eventual goal of performing a risk assessment. A great deal of the research performed in the field is geared toward the determination of the risk of producing a new product or releasing a pesticide or effluent to the environment. Because of the interaction between environmental toxicology and risk assessment, a basic and clear understanding of ecological risk assessment in necessary. Appendix B contains a reprint of the recent U.S. EPA document “A Framework for Ecological Risk Assessment”. This document is a relatively clear review of the basics of ecological risk assessment as percieved in the early 1990s. Since the original publication of this framework additional case studies and a guidance document have been published (U.S. EPA 1993, 1996). This chapter reviews the structure of ecological risk assessment and introduces some current developments. The latter sections also provide a suggested approach for the risk assessment of wide-area sites with multiple stressors. Two points should be considered carefully as regards the relationship between environmental toxicology and risk assessment. First, environmenal toxicology should not be seen as dependent upon risk assessment as its justification. Risk assessment is a management tool used for making decisions, often with a great deal of uncertainty. The science of environmental toxicology, as with any science, attempts to answer specific questions. In the case of environmental toxicology the question is primarily how xenobiotics interact with the components of ecological systems. Second, risk assessment is not a strictly scientific pursuit. The assessment endpoints of risk assessment are often set by societal perceptions and values. Although the scientific process may be used in the gathering of information in the assignment of risks, unless a testable hypothesis can be formulated, the scientific method is not being applied. As a management tool, risk assessment has certainly demonstrated its worth in the past 15 years. Table 11.1 Comparison of Hazard Assessment with Risk Assessment Characteristic Probabilistic results Hazard assessment Risk assessment No Yes Scales of results Basis for regulation Assessment endpoints Expression of contamination Tiered assessment Decision criteria Use of models Dichotomous Scientific judgment Not explicit Concentration Necessary Judgment Deterministic fate Continuous Risk Management Explicit Exposure Unnecessary Formal criteria Probabilistic exposure and effects Note: The primary distinguishing characteristic of risk assessment is its emphasis upon prob-abilistic criteria and explict assessment endpoints. Both methods of assessing the impact of toxicants are in use, but with risk assessment becoming the current standard. After Suter, G.W., II. 1990. In Aquatic Toxicology and Risk Assessment: 15th Volume. ASTM STP 1096. W.G. Landis and W.H. van der Schalie, Eds., American Society for Testing and Materials, Philadelphia, PA, pp. 5-15. BASICS OF RISK ASSESSMENT Perhaps the easiest definition of ecological risk assessment is the probability of an effect occurring to an ecological system. Note that the word “probability” is key here. Important components of a risk assessment are the estimations of hazard and exposure due to a stressor. A stressor is a substance, circumstance, or energy field that causes impacts, either positive or negative, upon a biological system. Stressors could be as wide ranging as chemical effects, ionizing radiation, or rapid changes in temperature. Hazard is the potential of a stressor to cause particular effects upon a biological system. The determination of an LD50 or the mutagenicity of a material are attempts to estimate the hazard posed by a stressor. Exposure is a measure of the concentrations or persistence of a stressor within the defined system. Exposure can be expressed as a dose, but in environmental toxicology it is often possible to measure environmental concentration. One of the values of determining tissue concentrations in fish and mammals is that it is possible to estimate the actual dose of a chemical to the organism. Biomarkers also may provide clues to dosage. A stressor posses no risk to an environment unless there is exposure. This is an extremely crucial point. Virtually all materials have as a characteristic some biolog-ical effect. However, unless enough of the stressor interacts with biological systems, no effects can occur. Risk is a combination of exposure and effects expressed as a probability. In contrast, hazard assessment does not deal with concentration and is not probabilistic in nature. Table 11-1 compares the two assessments as outlined in Suter (1990). Figure 11.1 Classical risk assessment paradigm. Originally developed for human health risk assessment, this framework does not include the close interaction between effects and exposure in ecosystems. ECOLOGICAL RISK ASSESSMENT Two basic frameworks for ecological risk assessment have been proposed over the past 10 years. The first was based on the National Academy of Sciences’ report detailing risk assessments for federal agencies. Though simple, this framework forms the basis of human health and ecological risk assessments. Even later refinements owe a great deal to this basic description of the risk assessment process. A diagram of the basic format is presented in Figure 11.1. Basically, four boxes contain the critical steps in the risk assessment. First, conceptual framework determines the specific questions that are to be asked during the risk assessment process. Second, the hazard assessment details the biological effects of the stressor under examination. Simultaneously, the exposure potential of the material to the critical biological components is calculated as part of an exposure assessment. Lastly, the probabilistic determination of the likelihood of an effect is formalized as risk characterization. Recently, the original framework was updated to specifically apply to estimating the risks of stressors to ecological systems. Perhaps of singular importance is the fact that exposure and hazard are not easily separated in ecological systems. When considering effects upon single organisms it is usually easy to separate exposure and effect terms. However, since ecosystems are comprised of many populations, the single species example is a subset of ecological risk assessment. For instance, once a chemical comes out of the pipe it has already entered the ecosystem. As the material is incorporated into the ecosystem biological and abiotic components trans-port or alter the structure of the original material. Even as the ecosystem is affected by the chemical, the ecosystem is altering the material. In light of this and other considerations a revised framework was presented in 1992. ECOLOGICAL RISK ASSESSMENT FRAMEWORK The ecological risk assessment framework attempts to incorporate refinements to the original ideas of risk assessment and apply them to the general case of ecological risk assessment. The overall structure is delineated in Figure 11.2. As before, the ecological risk assessment itself is characterized by a problem formulation process, analysis containing characterizations of exposure and effects, and a risk characterization process. Several outlying boxes serve to emphasize the importance of discussions during the problem formulation process between the risk assessor and the risk manager, and the critical nature of the acquisition of new data, verification of the risk assessment, and monitoring. The next few sections detail each aspect of this framework. Problem Formulation The problem formulation component of the risk assessment process is the begin-ning of a hopefully iterative process. This critical step defines the question under consideration and directly affects the scientific validity and policy-making usefulness of the risk assessment. Initiation of the process can begin due to numerous causes; for example, a request to introduce a new material into the environment, examination of cleanup options for a previously contaminated site, or as a component of exam-ining land-use options. The process of formulation is itself comprised of several subunits (Figure 11.3): discussion between the risk assessor and risk manager, stres-sor characteristics, identification of the ecosystems potentially at risk, ecological effects, endpoint selection, conceptual modeling, and input from data acquisition, verification, and monitoring. The discussion between the risk assessor and risk manager is crucial in helping to set the boundaries created by societal goals and scientific reality for the scope of the risk assessment. Often societal goals are presented in ambiguous terms such as protection of endangered species, protection of a fishery, or the even vaguer preserve the structure and function of an ecosystem. The interaction between the risk assessor and the risk manager can aid in consolidating these goals into definable components of a risk assessment. Stressor characteristics form an important aspect of the risk assessment process. Stressors can be biological, physical, or chemical in nature. Biological stressors could include the introduction of a new species or the application of degradative microorganisms. Physical stressors are generally thought of as a change in temper-ature, ionizing or nonionizing radiation, or geological processes. Chemical stressors generally constitute such materials as pesticides, industrial effluents, or waste streams from manufacturing processes. In the following discussion chemical stres-sors are used as the typical example, but often different classes of stressors occur Figure 11.2 Schematic of the framework for ecological risk assessment (U.S. EPA 1992). Especially important is the interaction between exposure and hazard and the inclusion of a data acquisition, verification, and monitoring component. Multivari-ate analyses will have a major impact upon the selection or assessment and measurement endpoints as well as playing a major role in the data acquisition, verification, and monitoring phase. together. Radionucleotides often produce ionizing radiation and also can produce toxic effects. Plutonium in not only radioactive but also is highly toxic. Stressors vary not only in their composition but in other characteristics derived in part from their use patterns. These characteristics are usually listed as intensity (concentration or dose), duration, frequency, timing, and scale. Duration, frequency, ... - tailieumienphi.vn